network: fix private sandbox netns #918

Job	Run time
tests (vmm/sandbox, --all-features)	3m 28s
tests (vmm/task, --all-features)	2m 40s
tests (shim, --all-features)	2m 28s
checks (vmm/sandbox, --all-features)	4m 47s
tests (quark, --all-features)	2m 31s
checks (vmm/task, --all-features)	3m 56s
tests (runc, --all-features)	2m 28s
checks (shim, --all-features)	3m 56s
tests (wasm, --features=wasmedge, 0.11.2)	2m 36s
checks (quark, --all-features)	3m 43s
tests (wasm, --features=wasmtime)	3m 21s
checks (runc, --all-features)	4m 8s
checks (wasm, --features=wasmedge, 0.11.2)	3m 48s
checks (wasm, --features=wasmtime)	4m 25s
	48m 15s

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